1. Technical Field
The present disclosure relates to a gamma control circuit and method thereof, and more particularly, to a gamma control circuit for adjusting a gamma curve by selecting a highest gamma voltage and a lowest gamma voltage from among a plurality of voltages, and a method thereof.
2. Discussion of the Related Art
A display device cannot display a true linear relationship between input image data and an output image and, thus, uses a gamma curve to display an optimum image by compensating for the nonlinear relationship and outputting the compensated image. However, a maximum value, a minimum value, and the slope of a gamma curve for the same image data are different according to the type of the display panel being employed.
For this reason, a gamma control circuit that provides various gamma curves is needed. However, the range of adjusting voltages with the existing gamma control circuit is limited and, thus, the integrated circuit chip size must be increased to make the range broader. Therefore, a gamma control circuit that is small in size but can provide various gamma curves, and a method of implementing same, are needed.
FIG. 1 is a block diagram of some constituent elements of a conventional display driving apparatus 100. Referring to FIG. 1, a decoder 120 of the display driving apparatus 100 receives input data and outputs gamma voltages, or, gray-scale voltages, corresponding to the input data, based on gray-scale voltages from a gamma control circuit 110.
If the input data is 6-bit data, the gamma control circuit 110 provides 64 gray-scale voltages V0 through V63. In this case, even if the same input data is supplied to the decoder 120, when a gray-scale voltage corresponding to the same input data is different, a voltage output from the decoder 120 is not the same. That is, a voltage output from the decoder 120 can be controlled by a gray-scale voltage. Accordingly, the gamma control circuit 110 is needed to control a gray-scale voltage according to the type of a display panel being used.
FIG. 2 is a circuit diagram of a conventional gamma control circuit 200. Referring to FIG. 2, control of the gamma voltages is performed by controlling reference voltages VREF1 through VREF8 corresponding to specific ones of the gray-scale voltages V0 through V63. Also, the gray-scale voltages V0 through V63 are controlled by controlling resistance values of variable resistors 214, 216, 234, and 236 in response to control signals C1 through C4, respectively.
Specifically, if the resistance value of the first variable resistor 214 is adjusted based on the first control signal C1, the first reference voltage VREF1, which is a reference voltage of the highest gray-scale voltage V0, is changed. That is, if a resistance value of the first variable resistor 214 is increased, the first reference voltage VREF1 is reduced, and thus, the highest gray-scale voltage V0 is also reduced. If the resistance value of the first variable resistor 214 is reduced, the first reference voltage VREF1 is increased, and thus, the highest gray-scale voltage V0 is increased.
Similarly, if the resistance value of the second variable resistor 216 is reduced in response to the second control signal C2, the eighth reference voltage VREF8 of the lowest gray-scale voltage V63 is reduced, and thus, the lowest gray-scale voltage V63 is reduced. If the resistance value of the second variable resistor 216 is increased, the eighth reference voltage VREF8 is increased, and thus, the lowest gray-scale voltage V63 is also increased. The shape of the whole gamma curve is determined by controlling the resistance of the third variable resistor 234 based on the third control signal C3 and the resistance value of the fourth variable resistor 236 based on the fourth control signal C4.
The first resistor array 252 connects the first and third variable resistors 214 and 234 to generate a plurality of voltages to be used as the second reference voltage VREF2, and one of the generated voltages is selected as the second reference voltage VREF2 in response to a first reference voltage control signal Q1 fed to a voltage selector 258.
The second resistor array 254 connects the third and fourth variable resistors 234 and 236 to generate a plurality of voltages, and a voltage selector selects and outputs the third through sixth reference voltages VREF3 through VREF6 in response to second through fifth reference voltage control signals Q2 through Q5, respectively, fed to the voltage selector 258.
The third resistor array 256 connects the second and fourth variable resistors 216 and 236 to generate a plurality of voltages, and the voltage selector outputs the seventh reference voltage VREF7 in response to a sixth reference voltage control signal Q6, fed to the voltage selector 258. For voltage stabilization, the reference voltages VREF1 through VREV8 are output via corresponding voltage followers, respectively.
The fourth resistor array 270 receives the second through seventh reference voltages VREF2 through VREF7, and outputs the gray-scale voltages V1 through V62, except for the highest and lowest gray-scale voltages V0 and V63.
FIG. 3 illustrates examples 300 of gamma curves that are controlled in response to first through fourth control signals C1 through C4, respectively. Referring to FIGS. 2 and 3, if the resistance value of the first variable resistor 214 is changed in response to the first control signal C1, the first reference voltage VREF1 and the highest gray-scale voltage V0 are changed, thus changing the inclination or slope of the gamma curve. If the resistance value of the second variable resistor 216 is changed in response to the second control signal C2, the lowest gray-scale voltage V63 is changed, thus changing the inclination of the whole gamma curve as illustrated in FIG. 3. If the resistance values of the third and fourth variable resistors 234 and 236 are changed in response to the third and fourth control signal C3 and C4, the highest and lowest gray-scale voltages V0 and V63 are not significantly changed but the intermediate reference voltages VREF2 through VREF7 are changed to change gray-scale voltages, thereby changing the inclination of the whole gamma curve as illustrated in FIG. 3.
FIG. 4 is a circuit diagram of a variable resistor 400 such as that employed in the circuit shown in FIG. 2. Referring to FIG. 4, the variable resistor 400 includes an array of resistors R1 through R4 and analog switches ASW1 through ASWn. The variable resistor 400 controls the overall resistance value by adjusting the number of resistors to be connected by switching on/off the analog switches ASW1 through ASWn in response to a control signals C1 through C4.
The range of adjusting voltages in response to the control signals C1 through C4 and reference voltage control signals Q1 through Q6 must be broad enough to provide various gray-scale voltages. Thus, the number of resistors of the variable resistor 400 and the number of analog switches must be increased to broaden the range, and the switch size must be very significantly increased to reduce resistance values of the analog switches. Also, if the resistance value of the first variable resistor 214 is changed, the whole resistance value is changed and, therefore, all of the reference voltages VREF1 through VREF8 are changed, thereby causing a user's inconvenience when performing gamma control.
As described above, a conventional gamma control circuit that uses variable resistors has a large chip size and is inconvenient to use when performing gamma control. Therefore, there is a need to develop a gamma control circuit that is small sized but can easily perform gamma control, while increasing the range of controlling gray-scale voltages, and a method performing gamma control.